Switch to Hugo

This commit includes all the needed workarounds and most changes from the main branch for everything to work, like:

* removing empty lines in raw HTML that break output
* read .browserslistrc, CSS variables from disk instead of duplicating it
* using Hugo mounts
* using Hugo for the docs CSS/JS
* move ToC Sass code to a separate file while adapting it for Hugo

Thus, this patch makes our npm scripts faster since lint runs on one step and there's no separate docs assets processing.

site/assets/scss/_sidebar.scss

@@ -0,0 +1,112 @@
+//
+// Left side navigation
+//
+
+.bd-sidebar {
+  order: 0;
+  // background-color: #f5f2f9;
+  border-bottom: 1px solid rgba(0, 0, 0, .1);
+
+  @include media-breakpoint-up(md) {
+    @supports (position: sticky) {
+      position: sticky;
+      top: 4rem;
+      z-index: 1000;
+      height: subtract(100vh, 4rem);
+    }
+    border-right: 1px solid rgba(0, 0, 0, .1);
+  }
+
+  @include media-breakpoint-up(xl) {
+    flex: 0 1 320px;
+  }
+}
+
+.bd-links {
+  width: 100%;
+  padding-top: 1rem;
+  padding-bottom: 1rem;
+  border-top: 1px solid rgba(0, 0, 0, .05);
+
+  @include media-breakpoint-up(md) {
+    @supports (position: sticky) {
+      max-height: subtract(100vh, 9rem);
+      overflow-y: auto;
+    }
+  }
+}
+
+.bd-search {
+  position: relative; // To contain the Algolia search
+  padding: 1rem 15px;
+  margin-right: -15px;
+  margin-left: -15px;
+
+  .form-control:focus {
+    border-color: $bd-purple-bright;
+    box-shadow: 0 0 0 3px rgba($bd-purple-bright, .25);
+  }
+}
+
+.bd-search-docs-toggle {
+  color: $gray-900;
+}
+
+.bd-sidenav {
+  display: none;
+}
+
+.bd-toc-link {
+  display: block;
+  padding: .25rem 1.5rem;
+  font-weight: 600;
+  color: rgba(0, 0, 0, .65);
+
+  &:hover {
+    color: rgba(0, 0, 0, .85);
+    text-decoration: none;
+  }
+}
+
+.bd-toc-item {
+  &.active {
+    margin-bottom: 1rem;
+
+    &:not(:first-child) {
+      margin-top: 1rem;
+    }
+
+    > .bd-toc-link {
+      color: rgba(0, 0, 0, .85);
+
+      &:hover {
+        background-color: transparent;
+      }
+    }
+
+    > .bd-sidenav {
+      display: block;
+    }
+  }
+}
+
+// All levels of nav
+.bd-sidebar .nav > li > a {
+  display: block;
+  padding: .25rem 1.5rem;
+  @include font-size(90%);
+  color: rgba(0, 0, 0, .65);
+}
+
+.bd-sidebar .nav > li > a:hover {
+  color: rgba(0, 0, 0, .85);
+  text-decoration: none;
+  background-color: transparent;
+}
+
+.bd-sidebar .nav > .active > a,
+.bd-sidebar .nav > .active:hover > a {
+  font-weight: 600;
+  color: rgba(0, 0, 0, .85);
+  background-color: transparent;
+}